Peptide aptamer for neutralizing the binding of platelet antigen specific antibodies and diagnostic and therapeutic applications containing the same

ABSTRACT

The present invention relates to a peptide aptamer which mimics particularly the human platelet antigen HPA-Ia epitope present on the platelet GPIIb/IIIa molecules and which is capable of neutralizing the binding of HPA-I a specific antibodies (anti-HPA-1 a). This peptide aptamer is advantageously used in a method for detecting and identifying HPA-I a specific antibodies in human serum, in a diagnostic kit for screening and identifying antibodies, in an immunoassay and a pharmaceutical composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/278,475, filed Oct. 15, 2008 (now U.S. Pat. No. 8,183,055), which isa 35 U.S.C. 371 National Application of PCT/EP2007/001042, filed Feb. 7,2007, which claims priority to European Patent Application Number06002496.5, filed Feb. 7, 2006.

DESCRIPTION

The present invention relates to a peptide aptamer which mimicsparticularly the human platelet antigen HPA-1a epitope present on theplatelet GPIIb/IIIa molecules and which is capable of neutralizing thebinding of HPA-1a specific antibodies (anti-HPA-1a). This peptideaptamer is advantageously used in a method for detecting and identifyingHPA-1a specific antibodies in human serum, in a diagnostic kit forscreening and identifying antibodies, in an immunoassay and apharmaceutical composition.

Platelets are one of the primary components of human blood and are thecellular components of the blood coagulation system. Blood is basicallymade up of plasma, red blood cells (erythrocytes), white blood cells(leukocytes), and platelets (thrombocytes). Platelets are produced inthe bone marrow by megakaryocytes. It is commonly understood thatplatelets are actually not true cells, but are fragments of membrane andcytoplasm containing granules. More specifically, platelets comprise anouter membrane and cytoplasm from megakaryocytes which in turn containgranules, dense bodies, a dense tubular system, and mitochondria.

Besides the platelet glycoproteins GPIIb/IIIa, GPIa/IX, GPIa/IIa orGPIV, platelets contain at least five glycoproteins that are linked byglycosyl phosphatidylinositol (GPI)-anchors. Among these is a 170-kDaglycoprotein which is classified as CD109. CD 109 is characterized bycarrying inter alia the Gov alloantigen.

Immune thrombocytopenia is a clinical status in which the platelets aredestroyed by antibodies. Idiopathic thrombocytopenia purpura (ITP) is acommon disorder characterized by autoimmune platelet destruction causedby IgG antiplatelet autoantibodies targeting GPIIb/IIIa, GPIa/IX,GPIa/IIIa or GPIV platelet glycoproteins. Neonatal/fetal alloimmunethrombocytopenia (NAIT) is caused by maternal human platelet alloantigen(HPA) alloantibodies (anti-HPA) which destroy fetal platelets provokinga severe thrombocytopenia. NAIT has an estimated incidence of 1/1000pregnancies and provides in utero cerebral bleeds or ventriculomegaly.The screening and the identification of alloantibodies is a mandatorystep to prevent and cure this disease. Post-transfusion purpura (PTP) isanother immune mediated destruction of platelets due to anti-HPAalloantibodies. Platelet refractoriness is a clinical situation in whichtransfused platelets are destroyed by alloantibodies produced by therecipient. The characterization of these antibodies is a necessary stepto provide efficient platelet transfusion.

Up to now, all methods for detecting auto- or alloantibodies directed toplatelets use fresh platelets in immunoblotting, immunoprecipitation,immunocapture, platelet immunofluorescent tests, or monoclonal antibodyimmobilization of platelet antigens such as monoclonal antibodyimmobilization platelet assay (MAIPA), or enzyme-linked immunosorbentassay (ELISA). All these assays require a collection of typed plateletsin the HPA systems beforehand. Moreover, these assays known in the artuse fresh platelets, as outlined above. However, during theirconservation at 4° C. or −20° C., the platelets' glycoproteins undergo ashedding process so that the platelet preparation is inappropriate todetect some platelet antibodies. All the attempts to keep a normal levelof the platelet glycoprotein expression for a longer period of time,such as more than one week, were not successful. Due to these facts, itis necessary to collect a new batch of platelets from the donors everyweek. Thus, there is a constant need in the field of platelet immunologyfor a method of keeping a normal level of platelet glycoproteinexpression during a long period of time (e.g. more than one month).

Thus, the technical problem underlying the present invention is toprovide a novel system enabling the detection and/or identification ofhuman platelet antigen specific antibodies in human serum. Inparticular, the present invention should provide (i) an aptamer whichretains its epitope expression of the platelet alloantigens present onplatelet glycoproteins,—i.e. the normal level and preferably also thenormal pattern—stably under storage, (ii) a process for producing suchstable aptamers, and (iii) analytical, especially immunologicalapplications of such stable aptamers.

The solution to the above technical problem is achieved by providing theembodiments characterized in the claims.

In particular, there is provided a peptide aptamer comprising:

-   -   (A) at least one dodecapeptide having in the direction from        N-terminus to C-terminus a sequence of        -   (i) four aliphatic amino acids selected from the group            consisting of valine, isoleucine, leucine, alanine and            glycine,        -   (ii) two acid amino acids selected from the group consisting            of aspartic acid and glutamic acid,        -   (iii) one proline,        -   (iv) one basic amino acid selected from the group consisting            of arginine, lysine and histidine,        -   (v) two acid amino acids selected from the group consisting            of aspartic acid and glutamic acid,        -   (vi) one alcohol amino acid selected from the group            consisting of threonine and serine and        -   (vii) one aromatic amino acid selected from the group            consisting of tryptophane, tyrosine and phenylalanine;    -   (B) at least one polypeptide; and optionally    -   (C) one or more detectable marker(s).

The term “peptide aptamer” as used herein also includes any compound asdefined above, wherein the dodecapeptide functions as a hapten. Further,the peptide aptamer according to the present invention is capable ofmimicking the HPA-1a epitope present on platelet glycoproteinsGPIIb/IIIa.

According to the present invention, the term “dodecapeptide” includesany dodecapeptides having an amino acid sequence as defined above orexplained in FIGS. 1 a and 1 b. In particular, the dodecapeptide may bebound to one or more polypeptides via any bonding mode, e.g. viacovalent, hydrogen bridge, van-der-Waals or electrostatic bond(s). In apreferred embodiment of the present invention, the dodecapeptide iscovalently linked to the polypeptide.

In one embodiment of the present invention, the peptide aptamer asdefined above comprises a dodecapeptide having the sequence VVAGDDPREDTW(SEQ ID NO: 1).

Further, herein, the term “polypeptide” is not limiting with respect tosequence length or type, but includes any suitable amino acid sequence,such as sequences of scaffold proteins or fusion proteins. According tothe present invention, said polypeptide(s) may also be used in thepresent invention in form of their fragments and may contain any residueoriginating from their preparation, such as labels used for detection orin any purification step. The polypeptide(s) according to the presentinvention also include(s) such polypeptide(s) having alterationsoriginating from contact with a biological environment, such asglycosylation, adhesion of nucleic acids or any other chemicalmodification.

In another embodiment of the present invention, the peptide aptamer asdefined above comprises a polypeptide selected from the group consistingof thioredoxin (Trx), green fluorescent protein (GFP), Staphylococcalnuclease, coiled coil polypeptides and cysteine-containing polypeptidesable to form disulfide bridges.

The polypeptide(s) according to the present invention may function as aninert carrier, a scaffold protein useful for presenting thedodecapeptide to the antibody, a purification aid useful for purifyingthe peptide aptamer or a detection means useful in detecting the peptideaptamer.

According to the present invention, the above defined dodecapeptide(s)may be bound to any position of the polypeptide(s). For example, thedodecapeptide(s) may be bound to the N-terminus or to the C-terminus ofthe polypeptide(s) or may be incorporated within the polypeptideresulting in two fragments flanking the N- and C-terminus of saiddodecapeptide. It is further possible that the peptide aptamer accordingto the present invention contains two or more dodecapeptides which maybe bound to the same or to different locations of said polypeptide(s) orto fragments thereof. In a further embodiment of the present invention,the peptide aptamer contains two or more dodecapeptides bound to eachother, directly or via a peptide spacer.

The term “detectable marker” as used herein is not restricted to aspecial type of detection marker, such as biochemical detection marker,but includes any residue known in the art which is suitable fordetection.

In one embodiment of the present invention, the peptide aptamer asdefined above, comprises a detectable marker selected from the groupconsisting of fluorescent marker, radioactive marker, His-tag,glutathione transferase (GST), Flag-tag, biotin, Ha-tag, Myc-tag andXPress-tag.

As used herein, the term “thioredoxin-human-platelet-antigen 1a”(Trx-HPA-1a) relates to a preferred embodiment of the present invention,wherein the peptide aptamer as defined above comprises a dodecapeptideaccording to SEQ ID NO:1, fused into the active site of the polypeptidethioredoxin (Trx), with said Trx having as a detection marker a His-tag(FIGS. 3 a and 3 b). Said Trx-HPA-1a has a protein sequence according toSEQ ID NO: 2 (FIG. 3 b) and is encoded by the nucleic acid sequence SEQID NO: 3 (FIG. 3 a).

The peptide aptamer of the present invention can be prepared by anymethod known in the art, such as recombinant DNA technology.

A further embodiment of the present invention relates to a method fordetecting or identifying human platelet antigen specific antibodies,comprising the steps of

-   -   (i) providing a human serum to be tested for human platelet        specific antibodies;    -   (ii) bringing into contact the human serum with the peptide        aptamer as defined above; and    -   (iii) detecting direct or indirect interaction of the peptide        aptamer with human platelet antigen specific antibody.

Herein, the term “human platelet antigen” includes the HPA-1a epitopepresent on the platelet GPIIb/IIIa glycoprotein but also the solubleHPA-1a epitope present on the soluble form of this GPIIb/IIIa in theserum or in any fluids of the body,

Herein, the term “human platelet antigen specific antibody” includes anyHPA-1a specific antibody (anti-HPA-1a) and also those antibodies whichhave a cross-reactivity with other proteins or glycoproteins.

As used herein, the term “human serum” includes any human blood serum,plasma or amniotic fluid obtained either directly from a patient or fromstored blood.

According to the present invention, the expression “direct or indirectinteraction” includes any interaction between the peptide aptamer andthe human platelet antigen specific antibody and further includes anyinteraction, wherein other molecules are involved. Such other moleculesmay be for example proteins, protein complexes, nucleic acids, sugarsand/or lipids.

The detecting step (iii) of the above-defined method comprises one ormore detection method(s) selected from the group consisting ofimmunoblotting, immunoprecipitation, immunocapture, monoclonal antibodyimmobilization of platelet antigens or enzyme linked immuno sorbentassay (ELISA), flow cytometry, protein array technology, spectroscopy,mass spectrometry, chromatography, surface plasmonic resonance,fluorescence extinction and/or fluorescence energy transfer.

In a preferred embodiment of the present invention, the antibody to bedetected or identified in the above-defined method is anti-HPA-1a.

In another embodiment of the present invention, the method as definedabove may be employed for detecting and/or identifying human plateletantigen specific antibodies in the serum of a patient suffering from aform of immune thrombocytopenia.

In a preferred embodiment of the present invention, the method asdefined above is used for detecting and/or identifying human plateletantigen specific antibodies in the serum of a patient, wherein thepatient suffers from immune thrombocytopenia, selected from the groupconsisting of ideopathic thrombocytopenia purpura (ITP), neonatal/fetalalloimmune thrombocytopenia (NAIT), post-transfusion purpura (PTP) orplatelet refractoriness.

A further embodiment of the present invention relates to a diagnostickit for screening and identifying human platelet antigen specificantibodies, comprising the peptide aptamer according to the presentinvention. The diagnostic kit may contain any furtheringredients/compounds known in the art for carrying out the screening oridentifying procedures.

Another embodiment of the present invention relates to an immunoassayfor the detection of human platelet antigen specific antibodies in humanserum, comprising the peptide aptamer according to the presentinvention. Examples of such immunoassays are immunoblotting,immunoprecipitation, immunocapture, monoclonal antibody immobilizationof platelet antigens, enzyme linked immuno sorbent assay (ELISA) and/orradioimmunoassay. Further assays relate to surface plasmonic resonance,fluorescence evanescence and flow cytometry.

A further embodiment according to the present invention relates to apharmaceutical composition, comprising a therapeutically effectiveamount of the peptide aptamer according to the present invention andoptionally one or more additional components selected from the groupconsisting of a pharmaceutically acceptable carrier, pharmaceuticallyacceptable salts, an auxiliary agent, a diluent and a solvent, or anycombination thereof.

Moreover, the pharmaceutical composition can be administered in asuitable dosage from 5 to 50 mg via any appropriate route such asparenterally, orally, cutaneously or sublingually or injected into alymphoid organ to a patient in need thereof.

The pharmaceutical composition of the present invention can be used forthe treatment and/or prevention of immune thrombocytopenia disorderssuch as ideopathic thrombocytopenia purpura (ITP), neonatal/fetalalloimmune thrombocytopenia (NAIT), post-transfusion purpura (PTP) orplatelet refractoriness. This aptamer could be used to neutralize theanti-HPA-1a antibodies in the plasma of mothers, it could be injectedinto the foetus or into the amniotic fluid. Moreover, this aptamer couldbe grafted on support like beads or polymers to deplete the antibodieswhen the plasma is incubated with these supports during an apheresisprocedure (selective plasma apheresis).

The figures show:

FIG. 1 shows (a) the amino acid combinations of the dodecapeptide (SEQID NO:1) comprised in the peptide aptamer according to the presentinvention. All sequences contain in this order: 4 aliphatic amino acids(V, I, L, A, G), 2 acid amino acids (D, E), 1 proline, 1 basic aminoacid (R, K, H), 2 acid amino acids (D, E), 1 alcohol amino acid (T, S)and 1 aromatic amino acid (W, Y, F). The arrow indicates the directionfrom N-terminus to C-terminus of the peptide. (b) a random example of adodecapeptide according to the present invention. The arrows indicatethe direction of sequence from N-terminus to C-terminus. The resultingdodecapeptide of the example has the peptide sequenceGly-Ile-Val-Val-Glu-Asp-Pro-Lys-Asp-Glu-Thr-Tyr (SEQ ID NO:4).

FIG. 2 shows Western blotting probed with the Camtran monoclonalantibody. Whole protein extracts from selected bacterial clones (1-9). Asample from HPA-1a platelets was used as a positive control (left lane,“plata”). Positive clones are characterized by a 63 kD band (lanes 2, 3,5, 8, 9).

FIG. 3 shows (a) the sequence (SEQ ID NO:3) encoding the Trx-HPA-1aprotein. This ORF is cloned between NdeI and SalI restriction sites ofthe pT7-7 expression vector. The Trx-HPA-1a protein possesses a 6-Histail at its C-terminal end. (b) the sequence (SEQ ID NO:2) of theTrx-HPA-1a protein. The protein contains a 12 amino acid long sequencein the active site of the thioredoxin, as well as a poly-histidine tailat the C-terminal end.

FIG. 4 shows (a) the detection of the recombinant protein by SDSPage(Coomassie blue). 20 μg of the protein purified by anion exchangechromatography were electrophoresed on a 20% polyacrylamide gel (lane1). Molecular weight markers are shown on the right. (b) theelectrospray mass spectrum of the Trx-HPA-1a protein. A major peak atexpected size (14 640 da) is detected. The minor peak (D) corresponds toTrx-HPA-1a lacking two methionone residues at the N-term end. (c) theMALDI-TOF mass spectrometry spectrum of the products obtained aftertryptic digestion of of Trx-HPA-1a. Black stars (H) correspond to peaksat expected size. Together these peaks correspond to 80% of theTrx-HPA-1a sequence.

FIG. 5 shows immunoprecipitates which were resuspended and analysed bywestern blotting using the anti-His antibody. Buffers used for theimmunoprecipitation were PBS (lanes 1, 2, 9, 10), PBS-Tween (0.01%)(lanes 3, 4), PBS-NP40 (0.1%) (lanes 5, 6), PBS-Triton (0.5% (lanes 7,8). Serum samples containing anti-HPA-1a antibody (1, 3, 5, 7) or not(2, 4, 6, 8) were used for immunoprecipitations. Negative controlswithout Trx-HPA-1a (9) or without immunoglobulins (10) are shown.

FIG. 6 shows the result of a typical immunocapture experiment. Thefollowing serums were used: two samples containing anti-HPA-1a antibody(1-2), one sample containing anti-HPA-1b antibody (3), one negativecontrol (4). Wells were coated with 50 or 100 ng of Trx-HPA-1a protein.Interaction with Trx-HPA-1a is only detected with serums containing theanti-HPA-1a antibody (1, 2).

FIG. 7 shows ELISA experiments. Time course analysis of OD at 492 nm.Serum containing (P+ and St Camb) (7 a) or not (429876, 383545, 489956,361177) (7 b) anti-HPA-1a antibody were used. The value of d²OD/dt² isshown (7 c). Calculation of d²OD/dt² allows to discriminate betweenpositive and negative samples.

FIG. 7 d shows the detection of HPA-1a specific antibodies in human serawhich was determined by an ELISA assay using microtitration plates(Nunc) coated overnight at 4° C. with 150 ng of Trx-HPA-1a per well incarbonate buffer. The plates were rinsed two times with 200 μL PBS.Nonspecific binding was blocked by placing 200 μL PBS containing 1%bovine serum albumin (BSA) (A7030, Sigma) for 100 minutes at roomtemperature (RT). Then, the plates were rinsed one time with 200 μL PBSand again incubated for 45 min at RT on shaking incubator with humansera diluted (1:100 to 4:5) in 100 μl PBS+1% BSA. After three washeswith PBS containing 0.05% Tween 20 and one washes with PBS, the plateswere incubated for 40 min at RT on a shaking incubator withHRP-conjugated Goat Anti-Human IgG, Fc fragment specific (JacksonImmunoResearch). After three washes with PBS+0.05% Tween 20 and twowashes with PBS, the binding of anti-HPA-1 antibody was detected byaddition of peroxidase substrate (OPD:1,2-phenylenediaminedihydrochloride) according to manufacturer's instructions(DakoCytomation). 20 min later, the reaction was stopped by adding 100μL 0.5 M H₂SO₄. The absorption was registered at 492 nm with ELISA platereader. The OD evolution at 492 nm depends on anti HPA-1a titer in theserum. This evolution is characteristic for the antibody-antigeninteraction, demonstrating that the Trx-HPA-1a aptamer can distinguishserum with or without anti-HPA-1a. Samples named Sch and Pan containedanti-HPA-1a antibody while sample Neg is a negative control from healthydonor. For high dilution ( 1/50) the OD is <0.05 for Neg. Serum whereasOD>0.23 for Sch and Pan. The situation is the same at low dilution (½)with the OD<0.11 for Neg. and >0.66 for Sch and Pan.

FIG. 8 shows (a) the neutralisation of alloantibodies anti-HPA-1a ofhuman serum in MAIPA assay. Two different dates of production were useto compare stability in time, (b) the neutralisation of Camtran antibodyin MAIPA assay with Trx-HPA-1a. 100 ng of Trx-HPA-1a is sufficient toinhibit Camtran in MAIPA assay and (c) the neutralisation of 2 humansera containing specific HPA-1a antibodies (anti-HPA-1a).

EXAMPLES Example 1

FliTrx™ Peptide Library and Monoclonal Antibody

The FliTrx™ random peptide library, based on the system described by Luet al. (Lu Z, Murray K S, Van Cleave V, LaVallie E R, Stahl M L, McCoy JM. Expression of thioredoxin random peptide libraries on the Escherichiacoli cell surface as functional fusions to flagellin: a system designedfor exploring protein-protein interactions. Biotechnology (N Y). 1995April; 13(4):366-72), was obtained from Invitrogen (San Diego, Calif.).A mAb Camtran against GPIIb/IIIa protein specific to phenotype HPA-1awas obtained from Cambridge (Griffin H. M., Ouwehand W. H., A humanmonoclonal antibody specific for the Leucine 33 (HPA-1a) form ofplatelet glycoprotein IIIa from V gene phage display library, Blood1995, 86, 12:4430-4436).

Growth and Induction of Peptide Library

Cell cultures and general panning methods are conducted as described inthe manufacturer's protocol. pFliTrx™ which contains the PLbacteriophage promoter to drive expression is propagated in E. coli(G1826) in which expression of the cl repressor is under the control ofthe trp promoter. The E. coli cells harboring the plasmids, are grown tosaturation overnight at 25° C. in IMC medium (1×M9 salts (40 mM Na₂HPO₄,20 mM KH₂PO₄, 8.5 mM NaCl, 20 mM NH₄Cl), 0.2% casamino acids, 0.5%glucose, 1 mM MgCl₂) containing 100 μg/ml ampicillin. Expression of thethioredoxin-flagellin fusion proteins containing the peptide inserts isinduced by 100 μg/ml tryptophan for 6 h at 25° C. A mixture of 0.1 g ofnon-fat dry milk, 300 μl of 5 M NaCl and 500 μl 20% α-methyl mannosideis then added to 10 ml of the induced E. coli culture. The resultingsolution is used as a peptide library ready for screening as follows.

Panning of the Random Peptide Display Library

60 mm tissue culture plates (Nunc) are used for peptide libraryscreening. Plates are precoated for one hour at 20-25° C. with 20 μg ofantibody diluted in 1 ml sterilized water. After the liquid is removed,the plates are washed with 10 ml sterile water and then supplementedwith 10 ml of blocking solution (1% non-fat dry milk, 150 mM NaCl, 1%α-methyl mannoside and 100 μg/ml ampicillin in IMC medium), gentlyagitating for 1 h. Just before the end of the 6 h induction of peptidelibrary, the blocking solution is decanted and 10 ml aliquots of theresulting solution are added to the plates. The plates are gentlyagitated at 75 rpm on a horizontal shaker for 1 min and incubated for 1h at 20-25° C. The bacterial culture is decanted and the plates arewashed by gently agitating for 5 min with 10 ml of IMC medium containing100 μg/ml ampicillin and 1% α-methyl mannoside. After washing anadditional four times, bound bacteria are detached with 1 ml IMC byvortexing for 30 s. The remaining detached bacteria are collected fromthe plate and grown for the next round of biopanning. The same procedureis performed in the subsequent four biopanning rounds. After five roundsof biopanning, the bacterial colonies are randomly picked from the RMG(1×M9 salts, 2% casamino acids, 0.5% glucose, 1 mM MgCl₂, 100 μg/mlampicillin and 1.5% agar) plates, and grown overnight at 30° C.

Western Blotting

Identification of positive clones by Western blotting is doneessentially according to the manufacturer's protocol. Briefly, fourtyclones of the RMG plate are transferred into 2 ml RM medium (1×M9 salts,2% casamino acids, 1% glycerol, 1 mM MgCl₂) containing 100 μg/mlampicillin), and grown to saturation at 30° C. with shaking. A 40 μlsample from the overnight culture is inoculated at 37° C. in 2 ml IMCcontaining 100 μg/ml ampicillin and 100 μg/ml tryptophan until the celldensity is OD 600 0.75. A 1.5 ml aliquot of induced cell culture isharvested and centrifuged at 10.000 g for 1 min. The pellet isresuspended in SDS-polyacrylamide gel-loading buffer, boiled for 5 minand electrophoresed in 8% SDS-polyacrylamide gel. The separated proteinsare blotted onto a nitrocellulose membrane (PROTRAN® BA79, Schleicher &Schuell) in a liquid electrophoretic transfer cell (Bio-Rad). Membranesare then blocked with TBS (10 mM Tris pH 7.2 and 0.15 M NaCl) 5% drymilk overnight at 4° C., then incubated with camtran antibody, diluted1:100 in TBS 1% dry milk, 0.05% Tween 20, for 2 h at 20-25° C. Afterbeing washed three times with TBS 0.05% Tween 20, membranes are thenincubated with horseradish peroxidase (HRP)-conjugated goat anti-humanIgG Fc specific (Sigma A0170) diluted 1:92000 for 40 min at 20-25° C.After a further three washes with TBS 0.05% Tween20, bound conjugate isdetected by Lumi-LightPLUS Western Blotting Substrate (Roche). Positiveclones are then re-analyzed by Western blotting carried out with a humanserum reacting against the GPIIb/IIIa protein which is specific to theHPA-1b phenotype.

DNA Sequencing

Plasmid DNAs of the identified clones are isolated using Wizard® Plus SVMinipreps DNA Purification System (Promega). The nucleotide sequencesare determined by MWG using the FliTrx™ Forward sequencing primer(5′-ATTCACCTGACTGACGAC-3′).

Cloning, Expression and Purification of Protein Trx-HPA-1a

Cloning and Generation of the Expression Plasmid

A cDNA encoding thioredoxin-peptide is amplified by PCR reaction usingthe pFlitrx plasmid selected previously. PCR conditions are as follows:1 min at 95° C., followed by 12 cycles at 95° C. 45 s, 36° C. 30 s, 72°C. 45 s, then 20 cycles 95° C. 45 s, 45° C. 30 s, 72° C. 45 s and 72° C.for 3 min to fill in the flush. At the 3′-end, the primer sequence5′-TGTCGACCAGGTTAGCGTC-3′ introduces a SalI site, and at the 5′-end, theprimer sequence 5′-TCATATGATGAGCGATAAAATTA-3′ introduces a NdeI site.The PCR generated DNA fragment is subcloned into the pGEM®-T Easy VectorSystem I (Promega). Then plasmid construct is amplified and digested byrestriction enzyme NdeI and SalI. The digested DNA fragment is clonedinto the NdeI and SalI sites of the ampicillin-resistant vector pT7-7 (avector based on the T7 polymerase expression system) encoding six Hiscodons followed by a stop codon downstream of the SaltI site. Theplasmid is transformed into Escherichia coli strain DHSa, andampicillin-resistant colonies are isolated. Resistant colonies are grownto allow large-scale production and purification of the plasmid. Properconstruction of the plasmid is confirmed by DNA sequencing.

Overexpression and Purification of Trx-HPA-1a (JT-PLP01)

The plasmid is transformed into E. coli strain C41(DE3) (Miroux B,Walker J E. Over-production of proteins in Escherichia coli: mutanthosts that allow synthesis of some membrane proteins and globularproteins at high levels. J Mol Biol. 1996 Jul. 19; 260(3):289-98). Afreshly transformed colony is inoculated into 400 mL of 2 YT medium (16%Bacto Tryptone, 10% Bacto YeastExtract, 85.5 mM NaCl) containing 20μg/ml ampicillin and grown at 37° C. to an OD600 of 0.6-0.8 beforeinduction with 0.7 mM isopropyl b-D thiogalactopyranoside. Afterovernight incubation at 37° C., cells are harvested by centrifugation.The pellet is suspended and incubated for 30 min at 4° C. in 10 mL oflysis buffer (20 mM Tris-HCl pH 8.0, 20% glycerol, 500 mM NaCl, 0.1%Triton X-100, 1 mM PMSF, 1 mg/mL lysozyme, ½ tablet Complete MiniEDTA-free (Roche) and 250 units/mL Benzonase (Merck)). Fractions (5 ml)are disrupted by sonication for 30 s. After four rounds ofcentrifugation of the supernatants (9000 g for 30 min), the solublefraction is applied onto an 7 ml Ni—NTA-agarose column (Qiagen)equilibrated previously in lysis buffer without lysozyme and Benzonase.Mixture is incubated in bath at 4° C., for 1 hour. The column is washedwith lysis buffer without lysozyme and Benzonase and then with 20 mMTris-HCl pH 8.0, 20% glycerol, 100 mM KCl, 0.5 mM PMSF, and 20 mMimidazole, Trx-HPA-1a is eluted with the same buffer containing 100 mMimidazole. Fractions containing Trx-HPA-1a as judged by SDS-PAGE arepooled. The buffer of the samples is exchanged and the sample isconcentrated by centrifugation with Vivaspin Concentrator membrane(Vivascience) cutoff of 5 kDa and 20 mM Tris pH 8.0, 10 mM NaCl. Samplesare applied to 1 mL Q-Sepharose columns (Mono Q HR 5/5, AmershamPharmacia Biotech). The column is washed with 20 mM Tris pH 8.0, 10 mMNaCl, and Trx-HPA-1a is eluted with a linear gradient of 10 mM to 1 MNaCl. Trx-HPA-1a rich fractions were pooled, concentrated to 1-15 mg/ml,equilibrated in PBS or sterilized water on Vivaspin Concentrator andstored at −20° C. until they are used. Protein concentrations aredetermined by the Bradford method (Kruger N J. The Bradford method forprotein quantitation. Methods Mol. Biol. 1994; 32:9-15).

Analysis of Purified Trx-HPA-1a

Mass spectra are acquired using an electrospray API 165 (AppliedBiosystems) instrument. The identity is established by peptide massfingerprinting and Maldi-Tof analysis (voyager—DE TM PRO, AppliedBiosystems).

Purification of Total IqG from Human Sera

The human sera equilibrated with 10% 1 M Tris-HCl pH 8 is applied ontoan 250 μl ml protein A-sepharose beads column (P3391, Sigma-Aldrich)equilibrated previously in 100 mM Tris-HCl pH 8. The column is washedwith ten volumes of 100 mM Tris-HCl pH 8 and ten volumes of 10 mMTris-HCl pH 8. Total IgG is eluted with 100 mM glycine pH 3. Elutionfractions are neutralized with 10% 1M Tris-HCl pH 8. Fractionscontaining IgG are concentrated and the buffer is exchanged on aVivaspin Concentrator membrane cutoff of 5 kDa and PBS. Samples areanalyzed in SDS-PAGE and IgG concentrations are determined by theBradford method (Kruger N J. The Bradford method for proteinquantitation. Methods Mol. Biol. 1994; 32:9-15).

Example 2

Co-Immunoprecipitation.

1 μg Trx-HPA-1a protein is incubated with 30 μg IgG extract from humansera in buffer A (PBS 0.5 M trehalose) or buffer A with 0.01% Tween 20or with 0.1% NP40 or with 0.05% Triton for 2 h at 20-25° C. 10% 1 MTris-HCl is added to the mixture and incubated with 25 μl of proteinA—sepharose beads previously equilibrated in 100 mM Tris-HCl pH 8 at 4°C. 45 min. The beads are then washed three times with 100 mM Tris-HCl pH8, 0.2% Tween 20 at 4° C. 10 min and then three times with 10 mMTris-Hcl pH 8, 0.2% Tween 20. To make theproteinA-sepharose-IgG-Trx-HPA-1a complex fall, the mixture iscentrifuged 2 min at 10 000 g. Proteins are eluted by boiling in SDSsample buffer and the eluate is resolved on a 15% SDS-polyacrylamide geland blotted onto a nitrocellulose membrane (PROTRAN® BA79, Schleicher &Schuell) in a liquid electrophoretic transfer cell. The membranes arethen blocked with TBS 5% dry milk at 4° C. overnight, then incubatedwith anti-His antibody (His-probe, Santa Cruz Biotechnology), diluted1:200 in TBS 1% dry milk, 0.05% Tween 20 for 1 h10 at 20-25° C. Afterthree washes with TBS 0.05% Tween 20, membranes are then incubated withHRP-conjugated goat anti-rabbit IgG (Dakocytomation) diluted 1:3000 for50 min at 20-25° C. After a further three washes with TBS 0.05% Tween20, bound conjugate is detected by Lumi-LightPLUS Western BlottingSubstrate.

Example 3

Immunocapture

Immunocapture is a classical technique to detect IgG antibodies directedagainst platelets according to the technique published by Rachel et al.(Med Lab Sci: 1985, 42; 194-199).

Herein, Interaction between Trx-HPA-1a and human sera containing Abagainst platelet HPA-1a is tested by immunocapture. A solid phase systemfor the detection of IgG Antibodies to platelets assay is used (CaptureP®, Immucor). The pool of platelets is replaced by Trx-HPA-1a protein.Microtitration plates with conics well are coated overnight at 4° C.with 50 ng or 100 ng of Trx-HPA-1a per well in carbonate buffer (15 mMNa₂CO₃, 35 mM NaHCO₃ in deionised water). After washing with PBS, 50 μlserum of anonymous donor is added into the wells and incubated at 37° C.for 45 min. After six washes, 50 μl of Capture-P indicating erythrocytes(red blood cell carrying anti-human IgG antibody).

Example 4

Immunoassay

The detection of HPA-1a specific antibodies in human sera is determinedby an ELISA test using microtitration plates (Nunc) coated overnight at4° C. with 100 ng of Trx-HPA-1a per well in carbonate buffer. The platesare rinsed two times with 250 μL PBS. Nonspecific sites are blocked by a1 h 30 min incubation at 20-25° C. with 250 μl PBS containing 1% bovineserum albumin (BSA) (A7030, Sigma). The plates are rinsed one time with250 μL PBS and then incubated for 45 min at 20-25° C. on a shakingincubator with 10 μl of human sera diluted in 100 μl PBS 1% BSA. Afterthree washes with PBS containing 0.05% Tween 20 and one wash with PBS,the plates are incubated for 40 min at 20-25° C. on a shaking incubatorwith HRP-conjugated Goat Anti-Human IgG, Fc fragment specific (JacksonImmunoResearch). After three washes with PBS 0.05% Tween 20 and twowashes with PBS, the binding of the anti-HPA-1 antibody is detected byaddition of peroxidase substrate OPD (1,2-phenylenediaminedihydrochloride) (DakoCytomation) according to manufacter's instruction.Absorption is measured at 492 nm and followed during 25-30 min usingELISA plate reader. To stop reaction 100 μl 0.5 M H₂SO₄ is added on thewell. Then the curve of evolution of absorbance according to theincubation's times of OPD is analyzed.

Example 5

In this experiment, the ability of Trx-HPA-1a to neutralize the bindingof human IgG HPA-1a specific polyclonal alloantibodies is determined bythe MAIPA technology. The MAIPA technology is used as described byKiefel et al. (Blood: 1987, 70; 1722-1727). The MAIPA technology is amore specific technique to detect and identify platelets' allo- andautoantibodies.

In particular, 50 μL of human serum containing or not polyclonalanti-HPA-1a or 15 ng of Camtran in 50 μL human serum without polyclonalanti-HPA-1a is incubated for 25 min at 20-25° C. with shaking incubatorwith 0, 1 ng, 10 ng, 100 ng or 1000 ng of Trx-HPA-1a diluted in 20 μLPBS. The mixture is preincubated 5 min with 3 μl of Ni—NTA agarose beadsand the complex alloantibodies/Trx-HPA-1a is pulled down bycentrifugation 13000 rpm, 2 min. 50 μl of supernatant is used to performthe MAIPA. The neutralization of serum is then determined as usual usingthe MAIPA technique.

Increasing concentrations of Trx-HPA-1a (0, 1 ng, 10 ng, 100 ng and 1000ng) diluted in 30 μL PBS are added to 50 μL of human serum containing ornot polyclonal anti-HPA-1a or 15 ng of Camtran diluted in 50 μL humanserum without polyclonal anti-HPA-1a. After incubation at 20-25° C. for25 min, the mixture is pre-incubated with 3 μl of Ni—agarose beads andthe complex alloantibodies or Camtran/Trx-HPA-1a is pulled down bycentrifugation at 13000 rpm for 2 min. The neutralization of 50μsupernatant serum is then determined as usual using the MAIPAtechnique. The results are shown in FIGS. 8 a, b and c.

Example 6

Detection of Anti-HPA-1a Antibodies Using MAIPA and ELISA

Blood samples (Table 1, left) are from anonymous donors or from NAITpatients. MAIPA is performed on either crude serum (Table 1, “MAIPA”) orprotein A-purified immunoglobulins (Table 1, “MAIPA pure”). The vastmajority of donors is found negative using both MAIPA and ELISA. Sevensamples found to be negative with MAIPA exhibit an optical density above0.1 (OD>0.1) with ELISA (end point measurement). However, for all donorsamples, time course analysis show that optical density increaseslinearly with time, the primary derivative being constant(dOD/dt=constant) and the secondary derivative being equal to zero(d²OD/dt²=0). Therefore a given sample is indexed as negative usingELISA if d²OD/dt²=0. Results of ELISA time course analyses are displayedin the indicated column (Table 1, “Curve”): donor samples are all testednegative (d²OD/dt²=0), while the three tested samples from NAIT patients(Pan, St Camb, Bru) are found positive (d²OD/dt²>0). Consequently, agiven sample is negative with the ELISA decribed herein if it fulfillsone of the following criteria: optical density below 0.1 (end point) ord²OD/dt²=0 (time course). It should be pointed out that all samplesfound to be negative with the end point protocol, are found negativewith the time course protocol as well. In addition, immuno-depletion ofthree samples from NAIT patients (Pan, Sch, Ac mB2) is successfullycarried out with the purified peptide aptamer (Table 1, “neutr.”).Genotyping for HPA locus is shown (Table 1, “geno.”).

Table 1 shows the summarized results from the anti-HPA-1a antibodydetection experiments using MAIPA and ELISA. The symbols are as follows:(−) negative; (+) positive; (−/+) first tested negative but positivelater on; (ND) not determined.

TABLE 1 Sample MAIPA Slope number MAIPA pure ELISA Curve neutr. geno.Donors: (n = 115)  1 to 103 − ND − ND 104 to 108 − ND − − 109 − − OD >0.1 − a/a 110 − − OD > 0.1 − a/a 111 − − OD > 0.1 − a/a 112 − ND OD >0.1 − ND 113 − − OD > 0.1 − a/a 114 − − OD > 0.1 − a/a 115 − − OD > 0.1− a/a Patients: (n = 27) Pen − − − ND Tro − − − ND Hal − − − ND Bra1 − −− ND Per − − − ND Ama − − − ND Del − − − ND Pas − − − ND Par − − − NDBra2 − − − ND Pie − − − ND Jas − − − ND Gar − − + ND LeM − + + ND DeA−/+ + + α 2b3a And − + + Mar − + + Bou − + + Cha − + + Pro + + + α glycoDum + + + α 3a Pan + + + + + b/b Kurc + + + St Camb + ND + + Sén + + −b/b Sch + + ND Bru + + + b/b Ac mB2 + + +

The invention claimed is:
 1. A nucleic acid containing a nucleotidesequence encoding a peptide aptamer comprising: (A) at least onedodecapeptide having in the direction of N-terminus to C-terminus asequence of SEQ ID NO: 1; (B) at least one polypeptide; and (C) one ormore detectable marker(s), wherein the peptide aptamer mimics humanplatelet antigen HPA-1a epitope present on glycoproteins GPIIb/IIIa. 2.The nucleic acid according to claim 1, wherein the polypeptide isselected from the group consisting of thioredoxin (Trx), greenfluorescent protein (GFP), Staphylococcal nuclease, coiled coilpolypeptides, cysteine-containing polypeptides that form disulfidebridges or one or more fragments thereof.
 3. A nucleic acid containing anucleotide sequence encoding a peptide aptamer comprising: (A) at leastone dodecapeptide having in the direction from N-terminus to C-terminusa sequence of: (i) four aliphatic amino acids selected from the groupconsisting of valine, isoleucine, leucine, alanine and glycine, (ii) twoacid amino acids selected from the group consisting of aspartic acid andglutamic acid, (iii) one proline, (iv) one basic amino acid selectedfrom the group consisting of arginine, lysine and histidine, (v) twoacid amino acids selected from the group consisting of aspartic acid andglutamic acid, (vi) one alcohol amino acid selected from the groupconsisting of threonine and serine and (vii) one aromatic amino acidselected from the group consisting of tryptophan, tyrosine andphenylalanine; (B) at least one polypeptide; and (C) one or moredetectable marker(s), the detectable marker(s) is/are selected from thegroup consisting of fluorescent marker, radioactive marker, His-tag,glutathione transferase (GST), Flag-tag, biotin, Ha-tag, Myc-tag andXPress-tag, wherein the peptide aptamer mimics human platelet antigenHPA-1a epitope present on platelet glycoproteins GPIIb/IIIa.